The DUP has called for greater cross-border intelligence sharing after MI5 warned there was a "strong possibility" of a dissident terrorist attack in Britain.

The security services yesterday raised the official threat level to England, Scotland and Wales from Northern Ireland-related terrorism to 'substantial' for the first time in three-and-a-half years.

Home Secretary Theresa May said the move "reflects the continuing threat from dissident republican activity".

In a statement to the House of Commons, she said: "As a result of this change, we are working closely with police and other relevant authorities to ensure appropriate security measures are in place."

The stark warning from the Home Secretary comes after two men were shot dead in Belfast in the last month, with a further 25 on a hit-list. And Prison officer Adrian Ismay died as a result of injuries he sustained in a dissident bomb attack in the east of the city in March.

It is understood that recent information uncovered by the intelligence services led to the threat level being raised from 'moderate' to 'substantial' in Britain.

'Substantial' is the third most serious category out of five, and means that a terrorist attack is seen as a strong possibility. The threat level from dissidents in Britain was last raised to 'substantial' in September 2010, and then lowered to 'moderate' in October 2012.

The last successful attack by dissidents on a British target took place some 15 years ago when the Real IRA planted a car bomb in Birmingham.

Mrs May said police were currently working to ensure "appropriate security measures are in place".

DUP MP Ian Paisley and Ulster Unionist MP Tom Elliott have both called for vigilance.

Mr Paisley described the move as "an extremely worrying development".

"There hasn't been an increase from Irish terrorism to the UK mainland for quite some time, but this shows that dissident republicans have now built up their capacity to hit targets on the mainland, and if they were successful, it would give them the PR campaign they crave," he said.

Mr Paisley also urged the Government to work with its counterparts in the Republic to "strengthen the weak link in the chain".

"I know the Government will be speaking to Irish authorities and I would urge them to work alongside the PSNI and Garda Siochana and provide whatever assistance they can," he said.

"We must all work together to strengthen the weak link in the chain - the Republic of Ireland - and ensure that the border is strengthened and vital information is shared between all relevant bodies across both sides of the border and across the Irish Sea".

Mr Elliott said the decision to raise the threat level was not one the security services or Government will have taken lightly.

"People need to know which organisations are now posing an increasing threat to the security of the UK and what level of co-operation there is between these groups," he said.

"Irish republican terrorists have murdered and created destruction for years. We don't want any more of these actions, it is time people were allowed to get on with their lives. Today's raising of the threat level is a reminder of the need for vigilance."

Mrs May made the announcement to the House of Commons yesterday.

"As a result of this change, we are working closely with the police and other relevant authorities to ensure appropriate security measures are in place," she said.

The threat level to the UK from international terrorism remains at 'severe', meaning an attack is highly likely. This has not been changed.

Mrs May said the threat level in Northern Ireland from terrorists also remains unchanged at 'severe'.

"The public should remain vigilant and report any suspicious activity to the police," she said, adding that the main focus of violent dissident republican activity continued to be in Northern Ireland "where they have targeted the brave police and prison officers who serve their communities day in and day out".

The Home Secretary added: "The reality is that they command little support. They do not represent the views or wishes of the vast majority of people, both in Northern Ireland and the Republic of Ireland, who decisively expressed their desire for peace in the 1998 Belfast Agreement and have been transforming Northern Ireland ever since.

"However, it is sensible, given their stated aims, that the public in Great Britain should also remain vigilant and report any suspicious activity to the police. But we should not be alarmed, and this should not affect how we go about our daily lives."

Meanwhile, it has emerged that 25 people in Belfast are currently under threat from dissidents.

North Belfast priest Father Gary Donegan said the threats were made against people accused by some individuals in their communities of "anti-social or criminal activity".

Fr Donegan has challenged those behind the shootings to step forward and explain themselves.

"The people that perpetrate the crimes are coming out of the shadows to do these things," he said.

"And they disappear back in again."

On Monday night takeaway deliver driver Dan Murray was murdered after being lured into a part of west Belfast by a bogus call in what was a third shooting in the city in 24 hours.

Last month taxi driver Michael McGibbon was killed in an alleyway shooting close to his north Belfast home.